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Computer Power Supply

The computer power supply is one of the components that was the least modified over the years. The first PC/XT power supply used in IBM computers were very big and offered only 63Watts of power. It was enough at first for systems working with floppy drives, but when the hard disk was introduced they also had to raise the maximum power output of the computer power supply to 130Watts.

The transition to AT form factor cases was very simple. Everything stayed the same except for the maximum power output and the toggle power switch. Power was raised to 200W and 250W on some models, but cables and connectors remained the same as on the first XT computer power supply, the only significant change was moving the power switch to the front of the computer case for easy access.

As you can see the AT power supply already had all the basic characteristics of the ATX power supplies we use today. Size, fan position and size, and most power cables remained the same.

More worrying is the power requirement of today’s computers. While most servers are satisfied with a 700-800Watt power supply, extreme gaming PC’s with up to 4 graphics cards require even more powerful power supplies that may reach 1200-1500Watts.

These high-power computer power supplies had to suffer some modifications, because such a big power consumption cannot be sustained a single circuit. Because graphics cards (and processors too) require 12Volts, they split it into 2 or more 12V circuits called Rails. This way they managed solve a more important problem too, that occurs when many different devices are connected to a single power circuit, so the power requirement is not stable anymore, some devices manage to disturb others with impulses they generate mostly on the 12V power cables. On power supplies with more rails, the CPU, graphics cards and other devices can be paired up in a way that they don’t exceed one rail’s maximum power specification and “noisy” devices don’t bother the quiet ones with impulses, because they are on different rails.

Unfortunately this device pairing on rails is not (yet) done automatically, usually the manufacturer decides and the user is left with very few combinations. In some more fortunate cases the manufacturer let’s the user decide, by putting connectors on the front of the power supply instead of soldering cables directly on the rails inside.

A few words need to be said about efficiency. While many people wonder why some power supplies are more expensive than others, technicians know exactly why, and computer stores will soon be forced to specify every power supply’s efficiency so the customer can decide. Just like in the field of home appliances, every fridge, air conditioner, washing machine and others have an unmistakable logo thar refers to the efficiency class A+,A,B,C,D … and so on. The cheapest Computer power supplies ($10 – $20) have an efficiency factor between 30% and 50%. This means that a 100Watts worth of components connected to such a power supply will consume 200-300Watts, thanks to the power supply’s losses.

More expensive power supplies ($25 – $50) have 60-76% efficiency factors, that are reasonable, but the best power supplies start from ~$100, and have efficiency factors above 80%, some manufacturers managed to go as high as 99% that don’t even need active cooling anymore.